This is the National Phase Application of PCT/NL00/00386, filed Jun. 7, 2000.
The invention relates to a process for regenerating periodic acid with a hypohalite and to a process of oxidising carbohydrates with periodic acid with regeneration and recovery of the periodic acid.
Periodic acid is extensively used for oxidising vicinal diols resulting in dialdehydes according to the following reaction:
R1xe2x80x94CHOHxe2x80x94CHOHxe2x80x94R2+IO4xe2x88x92xe2x86x92R1xe2x80x94CHxe2x95x90O+Oxe2x95x90CHxe2x80x94R1┤IO3xe2x88x92+H2O
Instead of periodate (IO4xe2x88x92), the actual oxidising species can be para- or meta-periodate (HnIO6(5xe2x88x92n)xe2x88x92), which formally results from addition of hydroxyl ions to the periodate ion. In the present description, all such oxoiodate species of heptavalent iodine, whether in neutral or en deprotonated form, are referred to as periodate. Usually, the two groups R1 and R2 are part of the same ring system, such as in carbohydrates. The most common process in which periodate is used as an oxidising agent is the oxidation of starch to dialdehyde starch (DAS), which is used as a wet strength additive for paper, or cain be further oxidised to calcium-binding dicarboxy starch.
As periodate is an expensive oxidising agent, the spent oxidising agent, i.e. iodate, should be regenerated to periodate. This is usually done by oxidation with hypochlorite, as described by McGuire and Meltretter, Die Starke, 23, (1971) 42-45. According to this method, spent iodate is treated with 1.5 eq of sodium hypochlorite at pH 13-14 and at 90-95xc2x0 C. for 40 min resulting in an average recovery of 97.6% periodate.
Although the prior art method already allows extensive regeneration of periodate, a loss of 2-3% of periodate for each oxidation cycle is still a problem for large-scale oxidation processes. Moreover, the regeneration conditions of the prior art method are rather severe (high temperature and high pH), thus adding to the cost of regeneration.
It was found according to the invention that the economy of the regeneration process of periodate using hypohalite can be improved by adjusting the aqueous medium of the regeneration so as to facilitate a physical separation of the regenerated periodate. The aqueous medium is adjusted by adding an agent that enhances precipitation of periodate, such as a water-miscible organic solvent or, in particular potassium ions and/or divalent ions such as calcium ions to the regeneration liquid.
An important advantage of the process of the invention is an increased yield of regenerated periodate. The yield is practically 100% (xe2x89xa799%). Such yields can be achieved at moderate pH, e.g. pH 10. Preferably the pH is between 7 and 11.5, most preferably between 9 and 11; it may be higher than 11.5, with even further enhanced yield, but with concomitant consumption of alkali. The temperature can also be moderate, i.e. below 80xc2x0 C. A temperature between 20 and 70xc2x0 C. can advantageously be used. Also, the high yields can be achieved using a relatively low amounts of hypochlorite (5-40% especially about 30% excess). As a further advantage, the oxidation product is free of iodate, whereas the prior art product usually contains traces of iodate.
The ions to be used include potassium and heavier alkaline metals ions such as rubidium and cesium, and divalent ions, e.g. alkaline earth ions or other ions such as zinc, lead and the like; preferred ions are potassium and the alkaline earth metals, such as strontium, barium and particularly calcium and magnesium; most preferred is potassium. The amount of these ions is at least 0.5 eq with respect to the iodate to be reoxidised (or to the periodate to be regenerated). In particular, the amount of ions such as potassium, calcium and/or magnesium is between 1 and 4 molar equivalents with respect to the iodate/-periodate. Other ions, in particular sodium ions, may also be present, but preferably less than 2 moles, especially less than 1 mole per mole of iodate and/or less than the equivalent amount of potassium and/or divalent ions. If necessary, the separation between periodate and oxidation product can be enhanced by addition of organic solvents such as alcohols to the oxidation medium.
Water-miscible organic solvents to be added include lower alcohols such as methanol, ethanol, 2-propanol and methoxyethanol, ethers such as dioxane and dimethoxy-ethane, ketones such as acetone, and the like.
The oxidising agent to be used for reoxidising the iodate is a hypohalite, including hypochlorite, hypobromite and, though somewhat less preferred, hypoiodite. The term hypohalite covers both ionic hypohalite salts such as sodium, potassium and calcium salts and undissociated hypohalites such as hypohalous acids and esters. Hypochlorite is preferred for economic reasons. The hypohalite may be added as such, or may be produced electrochemically, i.e. by passing an electric current through a solution containing an alkali metal halide or an alkaline earth metal halide.
The regenerated periodate is particularly suitable for the oxidation of carbohydrates to produce carbohydrate dialdehydes, such as members of the starch family (starch, amylose, amylopectin, hydrolysates and derivatives thereof), cellulose, other glucans, galactomannans (guar, locust bean gum), fructans (inulin), xylans, and the like, and alkylated, carboxyalkylated, hydroxyalkylated and other derivatives thereof provided they contain vicinal diol groups (xe2x80x94CHOHxe2x80x94CHOHxe2x80x94). Starch and starch derivatives and cellulose are especially preferred. It is observed that where the carbohydrate oxidation product is soluble in the reaction medium such as with low molecular weight carbohydrate dialdehydes, an effective separation of oxidation product (dialdehyde) and iodate can be achieved by precipitating the iodate, e.g. by addition of potassium, calcium or magnesium ions or an organic solvent such as ethanol. The precipitated iodate is then reoxidised by redissolution and oxidation as described above.
The dialdehyde oxidation products thus obtained can be used for various purposes, e.g. as a crosslinking agent, as an additive e.g. in glues, coatings, thickeners, and the like, or as a carrier material e.g. for proteins. They may also be used as starting material for producing dicarboxy carbohydrates, which are suitable e.g. as calcium binding agents. The dicarboxy carbohydrates can be conveniently prepared from the dialdehyde carbohydrates by oxidation with usual oxidising agents, in particular sodium chlorite. The dicarboxy starches that can be obtained from the dialdehyde starches prepared by using extended periodate oxidation periods (e.g. 4-8 days) have an unexpectedly high calcium-binding power, as measured by their sequestering capacity (SC) of at least 2.8, especially at least 3.0 mmol Ca/g.